Briefly, we have developed a comprehensive on line text-manipulation system. We wanted to determine the best means by which a user can designate textual entities to be used as "operands" in the different text-manipulation operations.

Techniques and devices for display-entity operand selcction represent a major component in any display control scheme, and are readily isolated for purposes of comparative testing, once the procedural environment in which selection is done has been established.

An important conclusion of our experimentation is that this environment has considerable effect upon the choice of display-selection means for a given display-control system.

To emphasize this, we point out that for two years we have been using the system for producing most of the internal memos* -- *and all of the proposals and reports -- associated with our research program.

This paper itself was extracted from one of these reports -- reorganized and modified by use of the system. See 1 (ENGLISH 1).

The format and writing style which represent an important experimental component of our research, are left in the form with which we work.

The user has generally been entering information on the typewriter-like keyboard.

To begin making the screen selection, his righl hand leaves the keyboard and takes hold of ("accesses," in our terminology) the selection device.

By moving this device he controls the position on the screen of an associated tracking mark (or "bug"), placing it over the "target" text entity.

He then actuates a pushbutton associated with the particular selection device, to tell the computer that he is now "pointing at" the target entity.

The computer puts a special mark under the entity which it determines as having been selected, to give the user an opportunity to see if a correct selection has been made.

Operand entities displayed on the screen are chosen by selecting a character within the operand entity (word, line, or statement).

The light pen or bug is first located near the desired character, then the SELECT switch on the device is depressed (or in the case of the knee control a special "CA" key on the keyboard is struck).

The Grafacon was manufactured by Data Equipment Company as a graphical input device for curve tracing (see Figure 2. See 2 (FLETCHER 1). The particular device that we tested is no longer marketed under this name. Data Equipment Company now markets the Rand Tablet under the name "Grafacon." See 3 (DAVIS 1).

It consists of an extensible arm connected to a linear potentiometer, with the housing for the linear potentiometer pivoted on an angular potentiometer.

A knob on the Grafacon arm is moved about by the user, and is depressed to activate the select switch (added by SRI) associated with the Grafacon.

However, certain features of the live working conditions were not closely related to the actual efficiency of the operand-selecting devices, such as

We tried either to eliminate these features from the experimental environment, or to fix them in some standard way through the experiment

A configuration simulating the "character mode" operation of the system consisted of nine x's, in a three by three array, with the array as a whole randomly placed on the display. The specific target entity was the middle x (see Figure 6(a)).

A configuration simulating the "word mode" operation of the system consisted of nine groups of five x's each, in a three by three "word" array, with the array as a whole randomly placed on the display. The target entity was any one of the five middle x's (i.e., any character in the middle " word"; see Figure 6(b)).

When the target appeared on the display screen, the subject was to strike the keyboard space-bar with his right hand, causing the bug to appear on the display. (Requiring that he use his right hand for both the space bar and the operand-selecting device made the experimental task closer to the actual on-line environment, where the user would often have both hands at the keyboard before moving to the operand selecting device. It also gave us a way of measuring the access times for the various devices.)

The subject was then to move his hand to the bug-positioning device being tested, and use it to guide the bug to the target entity on the display.

When the bug and the target coincided the subject was to "fix" the bug at that location, using the select switch of the bug-positioning device.

When the light pen rather than a bug-positioning device was used, the task sequence was much the same: after the target appeared, the subject was to strike the keyboard space bar with his right hand, then grasp the light pen and point it at the target entity (with the aid of the finder beam). The subject "fixed" his choice by depressing the select switch on the light pen. Correct and incorrect selections were signaled in the same way as with the bug-positioning devices.

For the experienced subjects, the entire testing procedure, which was broken into two time periods proceeded as follows:

For inexperienced subjects, the experimental procedure was somewhat different:

The position of the bug (in relation to the target entity) was recorded each 10 mulliseconds.

The times the subject hit the space bar, and the times he made either a correct or an incorrect entity selection, were recorded and appropriately tagged to aid in identifying these significant points in the late data analysis.

Tape-handling operations, controlled by commands from the on-line keyboard, facilitate searching through the data recorded on the magnetic tapes. These commands allowed one to scan forward or backward by one 32-target block of tests (or, an 8-target block, in the records for inexperienced subjects); and, within that block, to scan forward or backward one target (i.e. one presentation-selection event) at a time.

For each target-fix, the CRT could display a graph showing the bug's distance from its target entity as a function of time. This was displayed as two curves (see Figure 7), one showing variation with time of horizontal distance, and the other of vertical distance. The time-count was begun when the target appeared on the display. Vertical lines on the curves mark the time at which the space bar was struck and the time at which the target was correctly selected. Incorrect selections are shown as x's on the curve.

When studying a given target-fix event, the experimenter can, if he wishes, initiate output (to the on-line typewriter) of performance data: the time at which the space bar was struck, the time at which the bug movement began, the time at which the target was correctly selected, and the number of errors (incorrect selections) made. This software also computed and printed out the following incre mental times: the access time (from the time the space bar was struck until the time the bug movement began, measuring how long it took the subject to move his hand from the keyboard to the device); the motion time (from the time the bug began moving until the time the target was correctly selected); and total time (from the time the space bar was struck until the time the target was correctly selected -- i.e., the sum of access time plus motion time).

Finally, there is another command which causes the computer to search through a 32-target block of target fixes and compute (for output to the on-line typewriter) the average incremental times, and total number of errors, for that block.